[gcc r16-7190] c++/reflection: tweak eval_can_substitute

[Marek Polacek]

https://gcc.gnu.org/g:35d45295f555a4970e98180f4a2f6099f345c363

commit r16-7190-g35d45295f555a4970e98180f4a2f6099f345c363
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Jan 28 16:33:34 2026 -0500

    c++/reflection: tweak eval_can_substitute
    
    As discussed in
    <https://gcc.gnu.org/pipermail/gcc-patches/2026-January/705756.html>,
    we should check for undeduced_auto_decl after performing
    resolve_nondeduced_context_or_error.  Also add a test to exercise
    the new check.
    
    gcc/cp/ChangeLog:
    
            * reflect.cc (eval_can_substitute): Check undeduced_auto_decl after
            resolve_nondeduced_context_or_error.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/reflect/substitute3.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>
    Reviewed-by: Jakub Jelinek <jakub@redhat.com>

Diff:
---
 gcc/cp/reflect.cc                          |  2 +-
 gcc/testsuite/g++.dg/reflect/substitute3.C | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
index 78990ce414d2..3654f8814fb5 100644
--- a/gcc/cp/reflect.cc
+++ b/gcc/cp/reflect.cc
@@ -5339,7 +5339,7 @@ eval_can_substitute (location_t loc, const constexpr_ctx *ctx,
       if (fn == error_mark_node)
 	return boolean_false_node;
       fn = resolve_nondeduced_context_or_error (fn, tf_none);
-      if (fn == error_mark_node)
+      if (fn == error_mark_node || undeduced_auto_decl (fn))
 	return boolean_false_node;
       return boolean_true_node;
     }
diff --git a/gcc/testsuite/g++.dg/reflect/substitute3.C b/gcc/testsuite/g++.dg/reflect/substitute3.C
new file mode 100644
index 000000000000..ff1b1aeaf4da
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/substitute3.C
@@ -0,0 +1,22 @@
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+// Test std::meta::substitute.  Test the case when undeduced_auto_decl
+// is true after resolve_nondeduced_context.
+
+#include <meta>
+
+template<typename>
+auto
+f ()
+{
+}
+
+consteval bool
+g ()
+{
+  try { substitute (^^f, { ^^int }); }
+  catch (std::meta::exception &) { return false; }
+  return true;
+}
+
+static_assert (!g ());